PROJECT SUMMARY Naps benefit learning and memory in young children. However, children transition out of naps during the preschool years (~3-5 yrs). Whether naps should be encouraged in preschools or eliminated to provide more time for early learning is not clear. Reflecting this uncertainty, there are currently no formal recommendations regarding napping in childhood (e.g., from American Academy of Pediatrics). Our recent pilot study suggests the the transition out of naps may reflect maturation of the hippocampus, such that as this structure matures it allows for memories to be held in this short-term memory store across the day without detrimental interference. The specific objective of the proposed research is to examine the role of sleep and hippocampal development on memory during early childhood, specifically when children transition out of naps. The central hypothesis of this proposal is that maturation of the hippocampus during this period results in more information being retained without interference, reducing the need for frequent consolidation, which we posit underlies the transition out of naps. Preschool age participants will include both habitual nappers and habitual non-nappers. Using a within-subjects design, children will either be nap-promoted or wake-promoted mid-day. Children will participate in a visuospatial memory task prior to the nap/wake interval and recall will subsequently be reassessed. We will use an innovative combination of techniques to measure hippocampal volume and functional connectivity and sleep physiology during the nap. Aim 1 is to examine how the shift from biphasic to monophasic sleep is related to hippocampal structure and function. We hypothesize that, compared to habitual nappers, non-nappers will have better memory performance overall, less memory decay over intervals of wake, larger hippocampal volumes, and stronger connectivity within the hippocampal-cortical network. Aim 2 is to establish the relation between brain maturation and sleep on memory consolidation in early childhood. We hypothesize that brain maturation predicts memory consolidation over the nap. Importantly, we posit that brain maturation increases sleep spindles and slow wave activity which, in turn, increase nap-dependent memory consolidation. The translational significance may be seen in new policies regarding preschool nap opportunities and pediatric nap guidelines for preschool children. The theoretical significance is that these outcomes will drive an entirely new research dimension for educational sciences (sleep as a novel target to enhance learning) and spur further studies in the interaction of sleep and brain development on cognition.